This research project aimed to elucidate the developmental sequence of smooth muscle and the cytological mechanism involved in its growth and differentiation, selecting vascular smooth muscle of the 3rd branch of the superior mesenteric vesseles supplying the rat jejunum during perinatal period as an example.The length of the 3rd jejunal branches and the straight vessels encircling the jejunum were measured as the parameter of the longitudinal growth ratio of the vessels. Their luminal diameter was also measured in the mid-portion of the jejunal branches. Both parameters increased rapidly during the first postnatal 2 weeks and gradually by the 4 weeks thereafter. The changes in overall morphology of the vascular smooth muscle cells were examined with scanning electron microscopy after the removal of perivascular connective tissue components by tryptic digestion and/or NaOH hydrolysis. The shape, arrangement and size of vascular smooth muscle cells underwent rapid alterations during the first three postnatal weeks with a continuous increase in the medial thickness. There were marked differences in the morphological changes of smooth muscle cells between the mesenteric arteries and mesenteric veins. Cytoimmunochemical double staining with anti-alpha actin antigen and anti-proliferating cell nuclear antigen indicated that mitotic division of preexisting smooth muscle cells is the major source of the increment of the vascular medial thickness.